2020 Day 10

2020/day10.py

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+import numpy as np
+
+# Load input
+with open('input10', 'r') as f:
+  data = [0] + sorted([int(l) for l in f.readlines()])  # NB: We add the 0J source ourselves
+n_data = np.array(data)
+
+# Part 1: Connect all of them together
+diff = n_data[1:] - n_data[:-1]
+# Confirm this by multiplying the 1J differences with the 3J differences.
+# NB: we treat the internal adapter 3J difference as an off-by-one for simplicity.
+print('Part 1: ', (diff==1).sum() * (1 + (diff==3).sum()))
+
+# Part 2: Find the number of possible chains
+n_possibilities = {data[-1]: 1}  # The highest one can only connect to the internal adapter.
+for i in reversed(data[:-1]):  # We've already done the highest one as a special case
+  delta = n_data - i
+  p = 0
+  for n in n_data[np.logical_and(delta<=3, delta>0)]:  # You could probably plug two of the same together, but our input doesn't require us to handle that, and this simplifies things ;)
+    p += n_possibilities[n]
+  n_possibilities[i] = p
+print('Part 2: ', n_possibilities[0])

2020/input10

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+73
+114
+100
+122
+10
+141
+89
+70
+134
+2
+116
+30
+123
+81
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+142
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+167
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+32
+110
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+164
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+87
+4
+9
+107
+130
+19
+52
+84
+55
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+71
+83
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+72
+156
+41
+40
+1
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+158
+27
+31
+155
+25
+93
+166
+59
+108
+98
+149
+124
+65
+77
+88
+46
+14
+64
+39
+140
+95
+113
+54
+66
+137
+101
+22
+82
+21
+131
+109
+45
+150
+94
+36
+20
+33
+49
+146
+157
+99
+7
+53
+161
+115
+127
+152
+128